The invention is more particularly applicable to a seeding machine intended to sow light seeds, such as especially a vegetable seeding machine.
It is already known, in particular from U.S. Pat. No. 1,331,235, to pneumatically eject the seeds from the seeding machine distributor. This U.S. patent discloses a pneumatic seeding machine equipped with a distributor of the apertured disk type wherein the seed ejector system includes a blow-tube opening behind the seed transport apertures and a funnel provided with a guide duct disposed in front of said apertures and on the side of the disk opposite to the blow-tube. Since the size of the apertures in the distributor disk is adapted to the size of the seed to be sown, this known pneumatic ejector device have drawbacks in the case where it is intended to sow seeds of small size such as vegetable seeds. In fact, due to the small size of the distributor disk apertures, the air blown through the latter and carrying the seed onto the ground lacks strength and is unable to eject the seeds with sufficient force for, among others, rendering them insensitive to wind action.
Another type of pneumatic ejector is disclosed in French Pat. No. 1,378,012 which shows a mechanical distributor provided with sockets and cooperating with a pinion, the pinion teeth pushing the seeds out from said sockets. The pneumatic ejector disclosed in this patent mainly consists of a cylindrical tube fed at one end with compressed air and provided laterally with an inlet window formed under the sockets wherein the pinion teeth come to project. On this distributor, the seeds are pushed by the pinion teeth into the ejection tube and expelled towards the ground by the air flowing through said tube. Since the air pressure within the tube is higher than atmospheric pressure, a flow of leakage air escapes via the window through which the seeds are fed, so that this device is suitable only on distributors provided with members adapted to push the seeds into the tube, or else for distributing seeds sufficiently heavy for not being carried away by the leakage air-flow. However, such a pneumatic ejector is unsuitable for pneumatic distributors of the apertured disk type intended for sowing light seeds, since such seeds would be pushed back by the leakage air-flow crossing the window and would never work their way into the ejection tube.
German Pat. No. 429,529 describes the combination of a seed distributing device of the rotary drum type, of a venturi and of means for releasing the seeds at a distance from the venturi neck. A rotary drum carries mechanical seed-releasing device moving past a channel which opens at a venturi neck. The seeds are fed to the end of a suction channel and are then released. Since a compressed fluid flows through the converging and diverging sections of the venturi, a suction effect occurs in the lateral channel, so that the seeds which were released at a zero velocity at the outer end of the suction channel are gradually sucked in and accelerated up to the venturi neck. This system has several drawbacks. Firstly, the seeds are released far from the venturi neck. Thus, the suction created by the venturi has to be transmitted far from the latter, and so requires a good tightness between the venturi and the rotary cylindrical drum. Said tightness is difficult to obtain, since it should exist along a cylindrical surface. Secondly, the seed transport is not very efficient since the carrying force created by the suction flow is relatively weak. It should moreover be noted that, when the seeds are released, they have no velocity in the suction direction and would even rather have a velocity component of opposite direction. All of these factors jointly result in bad reliability of the system described in this patent.